Temperature sensitive Sdr mutants in which constitutive stable DNA Replication (SDR) is enhanced after a period of incubation at the restrictive temperature (42 degrees) were isolated. DNA synthesis in the mutants at 42 degrees appeared to be normal for as long as 3 hours during exponential growth. The SDR exhibited by the mutants was no more resistant to UV irradiation than normal replication, suggesting that it is not physiologically induced. Temperature-insensitive revertants isolated from one of the mutants lost the SDR capability, suggesting a single mutants affecting both the temperature sensitivity of growth and the SDR capability. The mutation was mapped in the same region (pro-lac) of the E. coli chromosome where the previously studied sdr mutations are located. The temperature sensitivity of growth was cell density-dependent and reversed by the presence of salts (NaCl, KCl, NH4Cl, Na2HPO4 at concentrations higher than 0.55). The results of preliminary experiments utilizing ethidiumbromide-CsCl centrifugation indicated that the Sdrc mutations previously studied (J. Mol. Biol., 121, 55) did not allow F plasmid DNA replication in the presence of chloramphenicol.